The invention relates to a dose setting device for a drug delivery system of the kind wherein a piston rod is successively pressed into a first end of a cylinder ampoule containing a drug to be delivered to press a piston closing said first end of the ampoule into the ampoule so that the drug is pressed out through a delivery member mounted at a second end of the ampoule. Dose setting is provided by rotating a second part of the piston rod in relation to a first part of said piston rod, which parts are connected by mating inner and outer threads on the respective parts, so that the total length of the piston rod is increased by a distance which is proportional with the extent of the rotation of the parts. When so rotated, a point on the second part of the piston rod is moved away from a stop position fixed in relation to a housing so that the set dose is delivered when said point is moved back to the stop position. The first part of the piston rod is guided in the housing so that it can be axially displaced but not rotated, and the second part of the piston rod remote from the piston is mounted so that it can be as well rotated as axially displaced.
EP 327 910 discloses a device by which doses may be set by increasing the total length of a piston rod, which abuts a piston of an ampule in the device, and a piston rod extension which is through a thread connection coupled to the piston rod. The increase is obtained by rotating the piston rod extension relative to the piston rod, by rotating a dose setting member which can be rotated relative to the housing and consequently relative to the piston rod. A connection between the dose setting member and the piston rod extension makes the piston rod extension follow the rotation of the dose setting member. By the resulting increase of the total length of the piston rod and its extension, the outer end of the piston rod extension, which was previously flush with the end of the housing is passed out through the end of the housing. The projecting end of the piston rod extension is used as an injection button which can be pressed until it again is flush wit the housing. The piston is thereby pressed into the ampoule a distance corresponding to the set increase of the total length of the piston rod and the piston rod extension. This setting and injection process can be repeated until the ampoule is empty and the piston rod and its extension have reached a maximal length, at which time the whole device is disposed of.
Similar dose setting devices are used in durable devices wherein only the ampoule is replaced by a new one when empty. Before a new ampoule can be mounted in the device, it is necessary to screw the two parts of the piston rod together to reduce the total length of said piston rod including its extension to the original length it had before the length was increased through repetitive dose settings. As mentioned, the two parts have to be screwed together manually, or the thread may be made with a pitch by which the angle of friction for the piston rod material is exceeded so that the parts, if allowed to, will rotate relative to each other and this way be screwed into each other when the parts are pressed axially towards each other. Also, the threads may be formed so that they may be drawn out of their mutual engagement when they are pressed towards each other. This axial pressing may be resulted by movement of a part of the housing of the device in order to obtain access to the space accommodating the ampoule.
In the device described in EP 327 910, the end of the piston rod projects from the end of the syringe a distance corresponding to the set dose. In the case of small doses, this distance may be very small, less than 1 mm. It may be desirable that the injection button has to be moved the same distance independent of the dose which is going to be injected.
A device wherein that wish is met is described in EP 235 312. In this device, an injection button with a push rod is reciprocated between fixed end positions. The push rod has a length allowing it just to abut the piston in the ampoule when the injection button is pressed home. If the length of the push rod is increased by a distance corresponding to a set dose when the rod is in its retracted position, it will, the next time the button is pressed home, press the piston into the ampoule a distance corresponding to the elongation provided by the setting of the dose. After the injection, the push rod will again be withdrawn from the piston. When the ampoule is empty, the push rod has been elongated to about twice its original length and before a new ampoule can be mounted in the device, the push rod must be screwed back to its original length. A drawback by this device is that the push rod is drawn away from the piston when the injection is finished and the button is no longer pressed. When this occurs, the piston, due to its elasticity and due to the pressure in the ampoule, may move backwards in the ampoule, whereby the just injected dose as well as the subsequent one are made imprecise.
Whereas injection devices so far has been given the shape of a fountain pen, a trend now points towards shorter devices which rather have the shape of a large lighter or a small pack of cigarettes. A reason for this development may be that ampoules with larger content, 3 ml instead of previously 1.5 ml, requires a pen diameter which makes it impossible to maintain the fountain pen illusion.
Whereas the device according to EP 327 910 could be provided with a flexible piston rod to adapt it to a short device, the device according to EP 245 312 is not suited for such adapting as the whole piston rod and its extension have to reciprocate in a guide which guides a flexible rod. Whereas the resistance against the movement of a flexible piston rod through a piston rod guide may serve as a barring against backwards moving of the piston rod, it will be unacceptable if the piston rod shall be reciprocated as reciprocation is conditioned on a reset spring which can overcome said resistance. When an injection is made the injection button must be pressed with a force which overcomes as well the resistance in the guide, the force of the spring; and the force necessary to press a liquid out from the ampoule and inject it.
It is an object of the invention to provide a dose setting device which is suited for short devices.
This is obtained by a device as described in the opening of this application, which device according to the invention is characterised in that the fist part of the piston rod is maintained in abutment with the piston in the cartridge, that the second part of the piston is coupled to a gear engaging a gear on a dose setting wheel to be rotated when said dose setting wheel is rotated, and that an injection mechanism is provided comprising a push button which can be moved between a projecting position and a pressed home position, and which injection mechanism has elements acting on the second part of the piston rod to press this second part to its stop position when the push button is pressed home.
In an embodiment of the device according to the invention the threads may be not self locking, the gear engaging the teeth of the dose setting wheel may be locked against unintentional rotation in relation to the housing, and the stop position may be defined by an end position of a surface of a lifter forming a part of the injection mechanism. The lifter reciprocates between fixed end positions, and said point of the second part of the piston rod may be defined as an endpoint at an end of this second part of the piston rod, which endpoint in abutment with said surface, forms a pivot about which the second part of the piston rod can rotate when not locked against such rotation.
When the threads connecting the two piston rod parts are not self locking, the two parts may be pressed together if they are allowed to rotate in relation to each other. However, the first part cannot rotate relative-to the housing and as the second part carries a gear engaging the teeth of a toothed dose setting wheel, which is again locked against unintentional rotation in relation to the housing, the two parts cannot be rotated relative to each other unless special precautions are taken. A movement of the injection member a constant distance each time an injection button is pressed is ensured by the provision of an injection member which can be reciprocated between fixed end positions. When this member is in its end position defined by the fact that an injection button is pressed home, a surface of the member defines the stop position to which a point of the second part of the piston rod is moved when the injection button is pressed home. The fact that said point is an end point at the outer end of the second part of the piston rod, which endpoint in abutment with said surface forms a pivot about which the second part of the piston rod can rotate when not locked against such rotation, ensures that the piston rod will either transmit axial forces from one end of the piston rod to the other or its parts will rotate so that the two parts are screwed together and the active piston rod is shortened. If the dose setting wheel is voluntarily rotated to set a dose, the two piston rod parts are rotated relative to each other to change the length of the piston rod in accordance with the relative rotation as the rotation of the dose setting wheel is transmitted to the second piston rod part through the gear.
According to an embodiment of a device according to the invention, a connection from a lid covering the ampoule may force the piston rod away from-the piston, unlock the dose setting wheel for rotation, and move the second part of the piston rod towards said surface of the lifter to make said end point of this second part abut said surface when the lid is opened. When the first part of the piston rod is drawn away from the piston, the nut member will be pressed towards said surface of the lifter and due to the threads being not self locking and due to the second part of the piston rod being freely rotatable the nut member will induce a rotation of said second part of the piston rod and this way be moved to the other end of said second part of the piston rod to be ready for a new series of dose settings and injections.
In a preferred embodiment of a device according to the invention, the threads may be self locking, the stop position may be defined by a beam fixed in the housing which beam may be abutted by a member fixed to the second part of the piston rod, and the nut member may be so designed that its thread can be coupled free from its engagement with the thread of the second part of the piston rod.
Self locking threads may have a smaller pitch than threads which are not self locking and a lower pitched thread gives a more precise setting of a dose. With self locking threads rotation of the second part of the piston rod does not occur when the nut element is moved along this part, and the nut element has to be de-coupled from its engagement with the thread on the second part of the piston rod in order to be moved along said second part without this part being rotated. This movement may Appropriately be induce by a connection from the lid covering the ampoule when said lid is opened to replace an empty ampoule.
To obtain said de-coupling, this nut member may have two intersecting bores of which one bore has an inner thread matching the outer thread of the second part of the piston rod and the other bore is smooth and fits slidingly over the thread of the second part of the piston, the nut member being tiltably mounted relative to the first part of the piston rod so that the threaded bore is concentric with the second part of the piston rod during the dose setting and injection and is tilted by said connection which act on the nut member to bring the smooth bore to a position concentric with said second part of the piston during withdrawal of the piston rod.